Novel polyethylene films

ABSTRACT

Novel stretch and blown films are prepared based on copolymers of ethylene and alpha-olefins having (a) a density in the range 0.900 to 0.940 (b) an apparent Mw/Mn of 2-3.4 (c) I 21 /I 2  from 16 to 24 (d) activation energy of flow from 28 to 45 kJ/mol (e) a ratio Ea(HMW)/Ea(LMW) &gt;1.1, and (f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95. The films exhibit an excellent combination of strength and processability and are particularly suitable for use as either stretch films or blown films for use as heavy duty sacks. The preferred films show a dart impact of &gt;1100 g and MD elongations of &gt;500%.

The present invention relates to copolymers of ethylene andalpha-olefins in particular to low density copolymers and also to novelfilms produced from said copolymer having improved properties inparticular improved stretch and creep characteristics.

In recent years there have been many advances in the production ofpolyolefin copolymers due to the introduction of metallocene catalysts.Metallocene catalysts offer the advantage of generally higher activitythan traditional Ziegler catalysts and are usually described ascatalysts which are single-site in nature. Because of their single-sitenature the polyolefin copolymers produced by metallocene catalysts oftenare quite uniform in their molecular structure. For example, incomparison to traditional Ziegler produced materials, they haverelatively narrow molecular weight distributions (MWD) and narrow ShortChain Branching Distribution (SCBD).

Although certain properties of metallocene products are enhanced bynarrow MWD, difficulties are often encountered in the processing ofthese materials into useful articles and films relative to Zieglerproduced materials. In addition, the uniform nature of the SCBD ofmetallocene produced materials does not readily permit certainstructures to be obtained.

Recently a number of patents have published directed to the preparationof films based on low density polyethylenes prepared using metallocenecatalyst compositions.

WO 94/14855 discloses linear low density polyethylene (LLDPE) filmsprepared using a metallocene, alumoxane and a carrier. The metallocenecomponent is typically a bis (cyclopentadienyl) zirconium complexexemplified by bis (n-butylcyclopentadienyl) zirconium dichloride and isused together with methyl alumoxane supported on silica The LLDPE's aredescribed in the patent as having a narrow Mw/Mn of 2.5-3.0, a melt flowratio (MFR) of 15-25 and low zirconium residues.

WO 94/26816 also discloses films prepared from ethylene copolymershaving a narrow composition distribution. The copolymers are alsoprepared from traditional metallocenes (eg bis (1-methyl,3-n-butylcyclopentadienyl) zirconium dichloride and methylalumoxanedeposited on silica) and are also characterised in the patent as havinga narrow Mw/Mn values typically in the range 3-4 and in addition by avalue of Mz/Mw of less than 2.0.

However, it is recognised that the polymers produced from these types ofcatalyst system have deficiencies in processability due to their narrowMw/Mn. Various approaches have been proposed in order to overcome thisdeficiency. An effective method to regain processability in polymers ofnarrow Mw/Mn is by the use of certain catalysts which have the abilityto incorporate long chain branching (LCB) into the polymer molecularstructure. Such catalysts have been well described in the literature,illustrative examples being given in WO 93/08221 and EP-A-676421.

Furthermore, WO 97/44371 discloses polymers and films where long chainbranching is present, and the products have a particularly advantageousplacement of the comonomer within the polymer structure. Polymers areexemplified having both narrow and broad Mw/Mn, for example from 2.19 upto 6.0, and activation energy of flow, which is an indicator of LCB,from 7.39 to 19.2 kcal/mol (31.1 to 80.8kJ/mol). However, there are noexamples of polymers of narrow Mw/Mn, for example less than 3.4, whichalso have a low or moderate amount of LCB, as indicated by an activationenergy of flow less than 11.1kcal/mol (46.7kJ/mol).

We have now found that it is possible to prepare copolymers of ethyleneand alpha-olefins having narrow Mw/Ma and low or moderate amounts ofLCB. These polymers are suitable for many applications which will beknown to those skilled in the art, but in particular are advantageousfor preparing films with an excellent balance of processing, optical andmechanical properties.

In particular the present invention is particularly directed to stretchfilms with excellent cling properties and to blown films suitable foruse for heavy duty sacks.

Our copending application WO 00/68285 describes copolymers of ethyleneand an alpha olefin having 3 to 10 carbon atoms, said copolymers having

-   -   (a) a density in the range 0.900 to 0.940    -   (b) an apparent Mw/Mn of 2-3.4    -   (c) I₂₁/I₂ from 16 to 24    -   (d) activation energy of flow from 28 to 45 kJ/mol    -   (e) a ratio Ea(HMW)/Ea(LMW) >1.1, and    -   (f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.

These copolymers may be used to prepare the full range of productsnormally manufactured from polyethylene copolymer products in thedensity range 0.900 to 0.940 kg/m³. Examples of applications for thecopolymers include injection moulding, rotomoulding, extrusion intopipes, sheets, films, fibres, non-woven fabrics, cable coverings andother uses which will be known to those skilled in the art. areparticularly suitable for the production of films and sheets preparedusing traditional methods well known in the art. Examples of suchmethods are film blowing, film casting and orientation of the partiallycrystallised product. The films exhibit good processability, improvedoptical and mechanical properties and good heat sealing properties.

WO 00/68285 described blown films from such copolymers having hazeranging from 3 to 20, dart impact >100 g and hexane extractables in therange 0.1-1.5%. Such films also exhibited a MD tear strength in therange 106-210 g/25 μm.

The application of polyethylene films in stretch wrapping has beenconsiderably enhanced by the use of linear low density polyethylene(LLDPE) type products. When formed into a film for stretch wrapapplication, LLDPE products typically combine a high extensibility withgood mechanical properties to provide a wrapping or collation functionto be achieved in an economic and effective manner. In this respect,LLDPE has significant advantages over LDPE which, due to both itsbehaviour in extension and its mechanical performance, is not normallyregarded as a product of choice for stretch wrapping applications.

Application of stretch wrap films may be either by hand or by machine.The film may be either wrapped directly onto the article or articles tobe packaged, or it may undergo a pre-stretching operation prior towrapping. Pre-stretching typically enhances the mechanical property ofthe film and provides a more effective packaging and more efficientcoverage for a given unit mass of film. Hence the response of the filmto either a pre-stretch or the stretch applied during wrapping is animportant parameter affecting film performance. In particular for agiven film width and thickness the efficiency with which an object iswrapped is affected by the degree to which the film can be thinnedduring the stretching and the loss of film width which may occur at thesame time. The resistance to sudden impact events, puncture by sharpobjects and the ability to maintain a tension sufficient to maintain thepackage in the desired shape and configuration are also importantparameters.

A further requirement in many stretch wrapping applications is that thefilm displays a certain degree of adhesive or cling behaviour enabling afirm closure of the package to be achieved without resort to use ofadditional securing measures such as straps, glues or heat sealingoperations. For monolayer films, such adhesion may be provided by theintrinsic film properties or by using a “cling” additive in the filmformulation. An example of a cling additive which is widely used ispoly(isobutene) (PIB) which term is taken to include polybutenesproduced from mixed isomers of butene. For multi-layer films, it isrelatively easy to provide one or more surface layers which arespecifically formulated to provide cling. In general this method allowsa more flexible approach to film manufacture as choice of product forthe main body of the film may be made on the basis of mechanicalperformance and the surface layers can be specially formulated foradhesion. Those skilled in the art will appreciate the multiplicity andflexibility of the choices of possible film structures.

A further requirement for the film producer is that the fabrication ofthe film is made as easy as possible by the use of polyethylenes havingprocessing characteristics which allow film extrusion to be carried outas easily as possible. The use of a product of lower molecular weight orbroader molecular weight distribution provides easier processability,but normally at the expense of a reduction in mechanical performance ofthe film. Similarly the use of products such an LDPE containing longchain branches (LCB) may assist processability but at the expense ofstretchability in the subsequent wrapping process.

We now found that a particularly advantageous combination of filmproperties may be obtained by producing a stretch film from the novelcopolymers described in the aforementioned WO 00/68285. The films have aparticularly advantageous combination of properties, combining highimpact resistance with easy processability and good performance instretch wrapping and when combined with polyisobutene as a clingenhancer, the films show a particularly advantageous control of clingforce.

Thus according to the present invention there is provided a stretch filmcomprising a cling additive in amount >0.5% and having

-   -   (a) dart impact of >450 g    -   (b) MD tear strength of >190 g/25 μm    -   (c) MD elongation at break of >450%

said film comprising a copolymer of ethylene and an alpha-olefin havingfrom 3 to 10 carbons atoms, said copolymer having

-   -   (a) a density in the range 0.900 to 0.940    -   (b) an apparent Mw/Mn of 2-3.4    -   (c) I₂₁/I₂ from 16 to 24    -   (d) activation energy of flow from 28 to 45 kJ/mol    -   (e) a ratio Ea(HMW)/Ea(LMW) >1.1, and    -   (f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.

The preferred stretch films according to the present are those having adart impact of >600 g and most preferably >1100 g.

The preferred films show an elongation of >500%.

The cling additive may be present in amount >2% and most preferably inamount of greater than or equal to 4%.

The preferred cling additive is polyisobutene (PIB).

The novel stretch films of the present invention may also be utilised inmulti layer films, for example in 3-layer films wherein the other layerscomprise polymers of lower density or copolymers as described above.

When extruded into a stretch film by film blowing, the products of theinvention give produce films with a particularly advantageous balance ofproperties. The processability of the ethylene copolymers during thefilm production process is typically comparable if not better than anLLDPE type polymer produced from a ziegler catalyst. The processabilityis assessed from measures such as the melt pressure in extrusion, theoutput rate for a given set of extruder conditions and the motor load.Such processing performance allows these products to be a “drop-in” forexisting LLDPE grades of similar specification without having to makeexpensive changes to extrusion machinery or suffering a handicap interms of extrusion performance.

As regards mechanical performance the dart impact of the films is veryhigh compared to a ziegler product of similar specification, beingtypically more than 600 g and preferably more than 1100 g for a film ofthickness 25 μm for a product of melt index about 1 and density 917.Film elongation is maintained at more than 500% despite the presence ofLCB. It is important that the film can be stretched to 300% or morewithout fracturing.

Due to their unique structure, the films of the invention show anadvantageous behaviour whilst undergoing stretching that the film widthis not unduly reduced. For a pre-stretch of 70% the films retain over75% of their initial width, this property being retained during storageof the film roll for up to one month or more.

The films of the invention show a hi-cling force as assessed by a Thimonstretch wrapping machine. A particularly advantageous behaviour is thatthe cling force varies only weakly with the amount of PIB cling agentadded to the film. Hence there is a wide latitude for addition levels ofPIB to vary without causing either too much or too little cling todevelop in the film.

Good elongation combined with outstanding impact resistance providessignificant advantages in wrapping applications.

In the application of polyethylene copolymer products in blown films, akey performance compromise is the balance between the modulus of thefilm and its impact performance. In general, alterations to the polymerstructure such as increasing the crystallinity lead to increased modulusbut at the expense of reduced impact performance. The advent ofmetallocene catalysed products has lead to a redefinition of thisperformance compromise. It is generally acknowledged that blown filmsfrom copolymers produced from metallocene catalysts have a differentbalance of properties when compared to LLDPE type products produced bythe more well established ziegler catalysts. When comparing products ofthe same basic specification in melt index and density, the metalloceneproducts tend to have very high impact properties due to narrowmolecular weight distribution and reduced modulus due to homogeneity ofcomonomer distribution.

We have found that the copolymers of the present invention can offerincreased modulus and impact when compared to more conventional zieglerproducts while at the same time having no penalty in extrusionperformance. For a given balance of performance in impact and modulus,the creep performance of the inventive resins is also better thanconventional Ziegler products, as are the film optical properties.Sealing is also improved. Hence the resins of the invention show manyadvantages without displaying any disadvantage in processing.

A particular application of blown films is for use in heavy duty sacksfor example for use for fertilisers, plastic pellets, etc. Themechanical properties of stiffness, impact and creep resistance are ofprime importance for the suitability of the copolymer product. Becauseof the intrinsic high impact resistance, the stiffness of the copolymerscan be increased while maintaining a better impact resistance comparedwith conventional products. Also due to the superior SCBD of thecopolymers the cereep resistance (creep elongation) is significantlyimproved leading to advantages in handling of the filled bags andprovides a potential for significant downgauging while maintainingsimilar performance to reference proprietary products.

For this application the films of the present invention suitablycomprise copolymers of density >0.920.

Thus according to another aspect of the present invention there isprovided a blown film having

-   -   (a) dart impact of >450 g    -   (b) MD tear strength >190 g/25 μm    -   (c) MD elongation >450%        said film comprising a copolymer of ethylene and an alpha-olefin        having from 3 to 10 carbons atoms, said copolymer having    -   (a) a density >0.920;    -   (b) an apparent Mw/Mn of 2-3.4    -   (c) I₂₁/I₂ from 16 to 24    -   (d) activation energy of flow from 28 to 45 kJ/mol    -   (e) a ratio Ea(HMW)/Ea(LMW) >1.1, and    -   (g) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.

The preferred blown films according to this aspect of the presentinvention are those having a dart impact of >600 g and mostpreferably >1100 g.

The preferred blown films show an elongation of >500%.

The novel blown films of the present invention may suitably be utilisedin blends, for example with medium density polyethylenes.

The most preferred copolymers for use in the novel stretch films of thepresent invention are those having

-   -   (a) a density in the range 0.900 to 0.940    -   (b) an apparent Mw/Mn in the range 2.5 to 3    -   (c) I₂₁/I₂ from 18-24    -   (d) activation energy of flow from 30 to 35 kJ/mol    -   (e) a ratio Ea(HMW)/Ea(LMW) >1.2, and    -   (f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.

By apparent Mw/Mn is meant a value of Mw/Mn uncorrected for long chainbranching.

The significance of the parameters Ea(HMW)/Ea(LMW) and g′(HMW)/g′(LMW)is described below. The experimental procedures for their measurementsare described later in the text.

The polymers contain an amount of LCB which is clearly visible bytechniques such as GPC/viscometry and flow activation energy. Thecontent of LCB is lower than reported in many earlier publications, butis still sufficient, when coupled with broadened Mw/Mn, to give improvedprocessability compared to linear polymers of narrow MWD (Mw/Mn lessthan about 3), which do not contain LCB.

For the measurement of LCB, we have found that the most usefultechniques are those which have a particular sensitivity to the presenceof LCB in the high molecular weight chains. For these high molecularweight molecules, the physical effects of LCB on, the solution and meltproperties of the polymer are maximised. Hence detection of LCB usingmethods based upon solution and melt properties is facilitated.

Activation energy of flow is commonly used as an indicator of thepresence of LCB in polyethylenes as summarised in the aforementioned WO97/44371. For lower amounts of LCB, for which the global activationenergy is of the order of 28 to 45 kJ/mol, it is found that the LCB hasa strong effect upon the activation energy as measured at low test ratesie the region in which the rheology is dominated by the high molecularweight (HMW) species. Therefore, the ratio- of activation energy derivedfrom the low rate data Ea(HMW) tends to exceed that derived from thehigh rate data, Ea(LMW). Hence polymers containing LCB predominantly inthe high molecular-weight chains tend to show the ratio Ea(HMW)/Ea(LMW)greater than unity.

A further well established method indicating the presence of LCB is gelpermeation chromatography with on-line detection of viscosity (GPC/OLV).By combining the data from 2 detectors, the ratio g′ can be derived as afunction of molecular weight; g′ is the ratio of the measured intrinsicviscosity [η] divided by the intrinsic viscosity [η]_(linear) of alinear polymer having the same molecular weight. In polymers containingLCB, the g′ measured at high molecular weights tends to be less thanthat measured at low molecular weights. To quantify this effect, we haveused a simple ratio g′(HMW)/g′(LMW). g′(HMW) is the weighted mean valueof g′ calculated for the 30% of the polymer having the highest molecularweight, while g′(LMW) is the weighted mean value of g′ calculated forthe 30% of the polymer having lowest molecular weight. For linearpolymers, g′ is equal to 1 at all molecular weights, and sog′(HMW)/g′(LMW) is also equal to 1 when there is no LCB present. Forpolymers containing LCB, g′(HMW)/g′(LMW) is less than 1. It should benoted that the g′ data can be corrected for the effect of short chainbranching (SCB). This would normally be done using a mean value of SCBcontent, the correction being applied uniformly at all molecularweights. Such a correction has not been applied here because inmeasuring the ratio g′(HMW)/g′(LMW) the same correction would apply toboth g′ values and there would be no net effect on the results reportedhere. Another method to quantify LCB content in polyethylenes is bycarbon-13 Nuclear Magnetic Resonance (13C-NMR). For the low amounts ofLCB observed for polymers of the invention it is generally-accepted thatthis technique can give a reliable quantification of the number of LCBpoints present in the polymer when the polymer is a homopolymer or acopolymer of ethylene and propylene or butene-1. For the purposes ofthis specification, a measurement of LCB by 13C-NMR is achieved in suchpolymers by quantification of the isolated peak at about 38.3ppmcorresponding to the CH carbon of a tri-functional long chain branch. Atri-functional long chain branch is taken to mean a structure for whichat least the first four carbon atoms of each of the 3 chains radiatingfrom the CH branch carbon are all present as CH2 groups. Care must beexercised in making such measures to ensure that sufficient signal:noiseis obtained to quantify the resonance and that spurious LCB structuresare not generated during the sample heating by oxidation inducedfree-radical reactions.

The above described analysis of LCB by 13C-NMR is much more difficultwhen the copolymer contains hexene-1. This is because the resonancecorresponding to an LCB is very close to or overlapping that for the CHcarbon at the branch site of the n-butyl branch obtained from thiscomonomer. Unless the two CH resonances can be resolved, which isunlikely using NMR equipment currently available, LCB could only bedetermined for an ethylene/hexene-1 copolymer using the above describedtechnique if the amount of n-butyl branches was so low, in comparison tothe amount of LCB present, that it could either be ignored or a reliablesubtraction carried out on the CH resonance at about 38.3ppm.

Using the preferred catalyst system of the present invention anethylene/butene-1 copolymer containing 6.5 wt % butene-1 has beenprepared using a continuous gas phase reactor. This polymer contained0.12 LCB/10,000 total carbons using the 13C-NMR technique describedabove. The spectrum was obtained from a 600 MHz NMR spectrometer after912,000 scans. The polymer also contained 0.25 n-butyl branches/10,000total carbons. No detectable oxidation was observed during this analysiswith a limit of detection of approximately 0.05/10,000 total carbons.

Despite a relatively low average LCB content, it would be expected thatsuch polymers would show distinctly modified rheological behaviour incomparison with truly linear polymers. If the LCB is concentrated in themolecules of higher molecular weight, as is known to be the case, thenan average value of 0.12 LCB/10,000 total carbons in the whole polymercould correspond to about 0.3 or more LCB/10,000 for molecules ofmolecular weight about one million. Hence these molecules would beexpected to contain at least 2 LCB points per molecule, equivalent to abranched structure with 5 arms. Such molecules are known to display verydifferent rheological properties to linear molecules.

The preferred polymers of the invention also show quite low amounts ofvinyl unsaturation as determined by either infra-red spectroscopy orpreferably proton NMR. For a polymer of melt index (2.16 kg) about 1,values are less than 0.05 vinyl groups per 1000 carbon atoms or even aslow as less than 0.02 vinyl groups per 1000 carbon atoms. Again, formelt index (2.16 kg) about 1, total unsaturations are also low comparedto some other metallocene polymers containing LCB, the totalunsaturations as measured by proton NMR to be the sum of vinyl,vinylidene, tri-substituted and cis+trans di-substituted internalunsaturation being in the range of less than 0.2 to 0.5 per 1000 carbonatoms. Products with higher or lower melt index, and hence lower orhigher number average molecular weights, may show respectively higher orlower terminal unsaturations, in proportion to the total number of chainends present. Hence the total unsaturations per 1000 carbon atoms areless than 17500/Mn where Mn is the number average molecular weightuncorrected for LCB and the vinyl unsaturations are less than 1750/Mn.

The comonomer present in the preferred polymers of the invention is notrandomly placed within the polymer structure. If the comonomer wasrandomly placed, it would be expected that the elution trace derivedfrom temperature rising elution fractionation (TREF) would show a singlenarrow peak, the melting endotherm as measured by differential scanningcalorimetry would also show a substantially singular and narrow peak. Itwould also be expected that little variation would be expected in eitherthe amount of comonomer measured as a function of molecular weight bytechniques such as GPC/FTIR, or the molecular weight of fractionsmeasured as a function of comonomer content by techniques such asTREF/DV. These techniques for structure determination are also describedin the aforementioned WO 97/44371, the relevant parts of which areincorporated herein by reference.

However, the comonomer may be placed in a way as to give a distinctbroadening of the TREF elution data, often with the appearance of one ortwo or even three peaks. At a polymer density of about 91.8 kg/m³ theTREF data typically show two main peaks, one at about 87° C. and anotherdistinct but smaller peak at about 72° C., the latter being about ⅔ ofthe height of the former. These peaks represent a heterogeneity in theamount of comonomer incorporated in the polymer chains. A third peak isoften visible at about 100° C. Without being bound by any theory thispeak is considered to be nothing other than a consequence of the factthat the polymer molecules of low comonomer content tend to crystalliseinto large chain folded crystals which melt and dissolve in the TREFexperiment in a narrow range of temperatures at about 100° C. The samepeak is very clearly visible in certain types of LLDPE polymers producedby ziegler catalysts and it is present in TREF analysis of MDPE and HDPEtype polyethylenes. Thus, without being bound by any theory, the thirdpeak at about 100° C. is more a result of the crystallisation of linearor near-linear molecules, than a feature which can be simply interpretedas representing a particular and separate polymer species.

The CDBI (Composition Distribution Branch Index) of the polymers isbetween 55 and 75%, preferably 60 to 75%, reflecting the fact that thepolymers are neither highly homogeneous (CDBI>about 90%) nor highlyheterogeneous (CDBI<about 40%). The CDBI of a polymer isreadily-calculated from techniques known in the art, such as, forexample, temperature rising elution fractionation (TREF) as described,for example, in Wild et al., Journal of Polymer Science, Polymer Phys.Ed., Vol 20, p 441 (1982), or in U.S. Pat. No. 4,798,081.

The behaviour seen in melting endotherms by DSC reflects the behaviourin TREF in that one, two or three peaks are typically seen. For examplethree peaks are often seen for the preferred polymers of density about918 kg/m³, when heated at 10° C./min. after crystallisation at the samerate. As is usual, it would be expected that the peaks seen in TREF andDSC would move to lower temperatures for polymers of lower density andto higher temperatures for polymer of higher density. The peak meltingtemperature Tp (the temperature in ° C. at which the maximum heat flowis observed during the second heating of the polymer) can beapproximated by the following expression within normal experimentalerrors:Tp=462×density−306The amount of comonomer measured as a function of molecular weight byGPC/FTIR for the preferred polymers shows an increase as molecularweight increases. The associated parameter C_(pf) is greater than 1.1.The measurement of C_(pf) is described in WO 97/44371.

The preferred copolymers exhibit extensional rheological behaviour, inparticular strain-hardening properties, consistent with the presence oflong chain branching.

The copolymers may suitably be prepared by use of a metallocene catalystsystem comprising, for example a traditional bisCp metallocene complexor a complex having a ‘constrained geometry’ configuration together witha suitable activator.

Suitable complexes, for example, are those disclosed in WO 95/00526 thedisclosure of which is incorporated herein by reference.

Suitable activators may comprise traditional aluminoxane or boroncompounds for example borates again disclosed in the aforementioned WO95/00526.

Preferred metallocene complexes for use in the preparation of thecopolymers may be represented by the general formula:

wherein:—

-   -   R′ each occurrence is independently selected from hydrogen,        hydrocarbyl, silyl, germyl, halo, cyano, and combinations        thereof, said R′ having up to 20 nonhydrogen atoms, and        optionally, two R′ groups (where R′ is not hydrogen, halo or        cyano) together form a divalent derivative thereof connected to        adjacent positions of the, cyclopentadienyl ring to form a fused        ring structure;    -   X is a neutral →⁴ bonded diene group having up to 30        non-hydrogen atoms, which forms a        complex with M;    -   Y is —O—, —S—, —NR*—, —PR*—,    -   M is titanium or zirconium in the +2 formal oxidation state;    -   Z* is SiR*₂, CR*₂, SiR*₂SIR*₂, CR*₂CR*₂, CR*═CR*, CR*₂SIR*₂, or        GeR*₂, wherein:    -   R* each occurrence is independently hydrogen, or a member        selected from hydrocarbyl, silyl, halogenated alkyl, halogenated        aryl, and combinations thereof, said R* having up to 10        non-hydrogen atoms, and optionally, two R* groups from Z* (when        R* is not hydrogen), or an R* group from Z* and an R* group from        Y form a ring system.

Examples of suitable X groups includes-trans-→⁴-1,4-diphenyl-1,3-butadiene,s-trans-→⁴-3-methyl-1,3-pentadiene; s-trans-→⁴-2,4-hexadiene;s-trans-→⁴-1,3-pentadiene; s-trans-→⁴-1,4-ditolyl-1,3-butadiene;s-trans-→⁴-1,4-bis(trimethylsilyl)i1,3-butadiene;s-cis-_⁴-3-methyl-1,3-pentadiene; s-cis-;⁴-1,4 dibenzyl-1,3-butadiene;s-cis-→⁴-1,3-pentadiene; s-cis-→⁴-1,4-bis(trimethylsilyl)-1,3-butadiene,said s-cis diene group forming a π-complex as defined herein with themetal.

Most preferably R′ is hydrogen, methyl, ethyl, propyl, butyl, pentyl,hexyl, benzyl, or phenyl or 2 R′ groups (except hydrogen) are linkedtogether, the entire C₅R′₄ group thereby being, for example, an indenyl,tetrahydroindenyl, fluorenyl, terahydrofluorenyl, or octahydrofluorenylgroup.

Highly preferred Y groups are nitrogen or phosphorus containing groupscontaining a group corresponding to the formula —N(R″) or —P(R″)—wherein R″ is C₁₋₁₀ hydrocarbyl.

Most preferred complexes are amidosilane- or amidoalkcanediyl complexes.

Most preferred complexes are those wherein M is titanium.

Specific complexes suitable for use in the preparation of the novelcopolymers of the present invention are those disclosed in theaforementioned WO 95/00526 and are incorporated herein by reference.

A particularly preferred complex for use in the preparation of the novelcopolymers of the present invention is (t-butylamido)(tetramethyl-→⁵-cyclopentadienyl) dimethylsilanetitanium-→⁴-1,3-penatadiene.

The activator may preferably be a boron compound for example a boratesuch as ammonium salts, in particular.

-   -   triethylammonium tetraphenylborate    -   triethylammonium tetraphenylborate,    -   tripropylammonium tetraphenylborate,    -   tri(n-butyl)ammonium tetraphenylborate,    -   tri(t-butyl)ammonium tetraphenylborate,    -   N,N-dimethylanilinium tetraphenylborate,    -   N,N-diethylanilinium tetraphenylborate,    -   trimethylammonium tetrakis(pentafluorophenyl) borate,    -   triethylammonium tetrakis(pentafluorophenyl) borate,    -   tripropylammonium tetrakis(pentafluorophenyl) borate,    -   tri(n-butyl)ammonium tetrakis(pentafluorophenyl) borate,    -   N,N-dimethylanilinium tetrakis(pentafluorophenyl) borate,    -   N,N-diethylanilinium tetrakis(pentafluorphenyl) borate.

Another type of activator suitable for use with the metallocenecomplexes are the reaction products of (A) ionic compounds comprising acation and an anion wherein the anion has at least one substituentcomprising a moiety having an active hydrogen and (B) an organometal ormetalloid compound wherein the metal or metalloid is from Groups 1-14 ofthe Periodic Table.

Suitable activators of this type are described in WO 98/27119 therelevant portions of which are incorporated herein by reference.

A particular preferred activator of this type is the reaction productobtained from alkylammonium tris(pentafluorophenyl) 4-(hydroxyphenyl)borates and trialkylaluminium. For example a preferred activator is thereaction product of bis(hydrogenated tallow alkyl) methyl ammonium tris(pentafluorophenyl) (4-hydroxyphenyl) borate and triethylaluminium.

The molar ratio of metallocene complex to activator employed in theprocess of the present invention may be in the range 1:10000 to 100:1. Apreferred range is from 1:5000 to 10:1 and most preferred from 1:10 to10:1.

The metallocene catalyst system is most suitably supported. Typicallythe support can be an organic or inorganic inert solid. Howeverparticularly porous supports such as talc, inorganic oxides and resinoussupport materials such as polyolefins which have well-known advantagesin catalysis are preferred. Suitable inorganic oxide materials whichmaybe used include Group 2, 13 14 or 15 metal oxides such as silica,alumina, silica-alumina and mixtures thereof.

Other inorganic oxides that may be employed either alone or incombination with the silica, alumina or silica-alumina are magnesia,titania or zirconia. Other suitable support materials may be employedsuch as finely divided polyolefins such as polyethylene.

The most preferred support material for use with the supported catalystsis silica. Suitable silicas include Crosfield ES70 and Grace Davison 948silicas.

The support material may be subjected to a heat treatment and/orchemical treatment to reduce the water content or the hydroxyl contentof the support material. Typically chemical dehydration agents arereactive metal hydrides, aluminium alkyls and halides. Prior to its usethe support material may be subjected to treatment at 100° C. to 1000°C. and preferably at 200 to 850° C. in an inert atmosphere under reducedpressure, for example, for 5 hrs.

The support material may be pretreated with an aluminium alkyl at atemperature of −20° C. to 150° C. and preferably at 20° C. to 100° C.

The pretreated support is preferably recovered before use in thepreparation of the supported catalysts.

The copolymers comprise copolymers of ethylene and alpha-olefins having3 to 10 carbon atoms. Preferred alpha olefins comprise 1-butene,1-hexene and 4-methyl-1-pentene. A particularly preferred alpha olefinis 1-hexene.

The copolymers are most suitably prepared in the gas phase in particularin a continuous process operating at a temperature >60° C. and mostpreferably at a temperature of 75° C. or above. The preferred process isone comprising a fluidised bed reactor. A particularly suitable gasphase process is that disclosed in EP 699213 incorporated herein byreference.

When prepared by use of the preferred catalyst systems described abovethe copolymers have a titanium content in the range 0.1 to 2.0 ppm.

EXAMPLES Catalyst Preparation

(i) Treatment of Silica

A suspension of Grace 948 silica (13 kg, previously calcined at 250° C.for 5 hours) in 110 litres (L) of hexane was made up in a 240 L vesselunder nitrogen. 1 L of a hexane solution containing 2 g/L of Stadis 425was added and stirred at room temperature for 5 minutes. 29.1 L of a892mmol Al/L solution of triethylaluminium (TEA) in hexane was addedslowly to the stirred suspension over 30 minutes, while maintaining thetemperature of the suspension at 30° C. The suspension was stirred for afurther 2 hours. The hexane was filtered, and the silica washed withhexane, so that the aluminium content in the final washing was less than0.5 mmol Al/litre. Finally the suspension was dried in vacuo at 60° C.to give a free flowing treated silica powder with residual solvent lessthan 0.5 wt %.

(ii) Catalyst Fabrication

All steps, unless otherwise stated, of the catalyst fabrication werecarried out at 20° C. 3 L of toluene was added to a 24 L vessel equippedwith a turbine stirrer, and stirred at 300 rpm. 5.01 L of a 9.5 wt %solution in toluene of bis(hydrogenated tallow alkyl) methyl ammoniumtris(pentafluorophenyl)(4-hydroxyphenyl)borate was added during 15minutes. Then 1.57 L of a 250mmolAl/L solution in toluene oftriethylaluminium was added during 15 minutes and mixture stirred for 30minutes. The solution obtained was then transferred under nitrogen, withstirring during 2 hours, to an 80 L vessel containing 10 kg of the TEAtreated silica described above. 60 L of hexane was then rapidlyintroduced and mixed for 30 minutes. 1.83 kg of a 7.15 wt % solution inheptane of (t-butylamido)(tetramethyl-η⁵-cyclopentadienyl)dimethylsilanetitanium-η⁴-1,3-pentadiene was added during 15 minutes.Mixing was continued for 1 hour and 1 L of a 2 g/L hexane solution ofstadis 425 was added. The catalyst slurry was then transferred to avessel of volume 240 L and 70 L of hexane added. Excess solvent wasremoved by decantation, and a further 130 L of hexane added. Thisprocess was repeated until less than 0.2 L of toluene remained in thesolvent. 1 L of a 2 g/L hexane solution of stadis 425 was then added andthe catalyst dried under vacuum at 40° C. to a residual solvent level of1 wt %.

(iii) Polymerisation Using Continuous Fluidised Bed Reactor

Example 1

Ethylene, 1-hexene, hydrogen and nitrogen were fed into a continuousfluidised bed reactor of diameter 45 cm. Polymerisation was performed inthe presence of a catalyst similar to that prepared above. Polymerproduct was continuously removed from the reactor. Operating conditionsare given in Table 1.

Example 2

The procedure for example 1 was scaled up to produce a catalyst of batchsize approximately 75 kg. This catalyst was used to produce a copolymerin a commercial gas phase scale reactor of diameter 5 metres again usingthe conditions shown in Table 1. TABLE 1 Example 1 2 total pressure(bar) 20.0 19.8 temperature (° C.) 80 75 ethylene pressure (bar) 7.5 8.1H₂/C₂ ratio 0.0025 0.0023 C₆/C₂ ratio 0.0055 0.0050 production (kg/hr)74 8700

Comparative Example 1

A film from Dowlex 2045 was used for comparison.

3 layer films were produced on a coextrusion operating line at about 100kg/hr. This line was equipped with 4 25 L/D LLDPE extruders and a 300 mmdiameter die with 1.2 mm die gap. The film was of thickness 25 mm andthe blow up ratio 2.5:1. The inner cling layer was formed from an EVAcopolymer containing TAC 100 (50% PIB). The other layers were formedfrom the test polymer containing TAC 100.

Details of the copolymers prepared and films produced are given in Table2. TABLE 2 Film properties Example Comp Comp Comp 1a 1b 1c 1a 1b 1c 2aMI/2.16 g/10 mn 0.91 0.91 0.91 1.18 1.18 1.18 1.3 HLMI g/10 mn 25.8 25.825.8 23.70 23.70 23.70 25.80 MFR 28.4 28.4 28.4 20.1 20.1 20.1 19.8Density kg/m³ 919.4 919.4 919.4 916.6 916.6 916.6 916.9 EXTRUSIONCONDITIONS Melt pressure bar 533 494 460 508 496 467 454 Melttemperature ° C. 232 232 231 229 233 230 228 Output kg/h 95 95 95 110110 110 110 Motor Load A 55 50 50 54 51 49 49 Blend 4% PIB 5% PIB 6% PIB4% PIB 5% PIB 6% PIB 5% PIB MECHANICAL PROPERTIES Dart Impact g 265 350310 >1100 >1100 >1100 >1100 Elmendorf tear MD g/25 μm 255 207 196 str.TD g/25 μm 656 577 572 Elongation at MD % 670 640 600 break TD % 780 660680

Example 3

A resin was produced in the gas phase using a similar catalyst system tothat described above with melt index 1 and density 923.6 kg/m³. This wasextruded into film 150 μm thick on a Reifenhauser blown film lineequipped with a die of diameter 150 mm and die gap 2.3 mm. The productwas extruded both pure and blended with 20% of a medium densitypolyethylene of density about 938 kg/m³, melt index about 0.2 producedusing a chromium catalyst system.

Comparative Example 2

Dowlex 2045 was used as a comparative example.

The blown film properties are given in Table 3 below. The films werealso tested in creep at 60° C. under 5Mpa load. After 200 minutes, thedeformation of the film of example 1b was 57% compared to 63% forcomparative example 2 TABLE 3 Example 1a 1b Comp 1 MI/2.16 g/10 mn 1.001.00 0.94 HLMI g/10 mn 23.46 23.46 26.8 MFR 23.5 23.5 28.5 Density kg/m³923.6 923.6 919.7 EXTRUSION Die mm 150 150 150 Die gap mm 2.3 2.3 2.3Screw speed rpm 83.4 85 89.2 Melt pressure bar 267 283 268 Melttemperature ° C. 216.7 217 217.1 Output kg/h 50 50 50 BUR 2:1 2:1 2:1Motor Load A 62 65 61 Specific energy KWh/Kg 0.22 0.23 0.23 Thickness μm150 150 150 Blend pure +20% MDPE +20% MDPE MECHANICAL PROPERTIES DartImpact g 1295 1084 890 Edge fold impact (Staircase Method) (g) 805 735650 Elmendorf tear str. MD g/25 μm 260 210 341 TD g/25 μm 418 471 573Tensile str. at yield MD MPa 12.9 14.4 12.5 TD MPa 14 14.6 13.4 Tensilestr. at break MD MPa 48 45.6 43.9 TD MPa 47.5 41.6 42.5 Elongation atbreak MD % 1250 862 930 TD % 1000 917 1000 Secant modulus 1% MD MPa 235263 208 TD MPa 285 298 239 Haze % 23.8 22.5 19.8 Gloss 45° ‰ 57.7 49.447.9

Methods of Test

Melt index (190/2.16) was measured according to ISO 1133.

Melt flow ratio (MFR) was calculated from the ratio of flow ratesdetermined according to ISO 1133 under condition (190/21.6) andcondition (190/2.16).

Density was measured using a density column according to ISO1872/1-1986, except that the melt index extrudates were not annealed butwere left to cool on a sheet of polymeric material for 30 minutes.

Dart impact was measured by ASTM D1709, tear strength by ASTM D1922, andhaze by ASTM D1003.

1-6. (canceled)
 7. A blown film having (a) dart impact of >450 g, (b) MDtear strength >190 g/25 μm, and (c) MD elongation >450% said filmcomprising a copolymer of ethylene and an alpha-olefin having from 3 to10 carbon atoms, said copolymer having (a) a density >0.920, (b) anapparent Mw/Mn of 2-3.4, (C) I₂₁/I₂ from 16 to 24, (d) activation energyof flow from 28 to 45 kJ/mol, (e) a ratio Ea(HMW)/Ea(LMW) >1.1, and (h)a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.
 8. A blown film according toclaim 7 having a dart impact >600 g.
 9. A blown film according to claim8 having a dart impact >1100 g.
 10. A blown film according to 7 having aMD elongation of >500 g.